“Congress did amend the National Defense Authorization Act three years ago to prohibit funding for the transfer of any Guantánamo detainee to the U.S. It also prohibited funding for transfers to other countries, unless the Defense Secretary personally certified that the transferred detainee would never engage in terrorist activity. Because no one can give such a personal assurance, that provision effectively blocked transfers. But Congress then amended the law to allow the Secretary to waive that requirement and to transfer detainees to other countries if he finds (1) that the receiving country will take steps to ‘substantially mitigate’ the risk that the detainee will engage in terrorist activity, and (2) that the transfer is in U.S. national security interests.”